The present invention relates to compander systems, and more particularly to such systems used for the transmission and reception of a multichannel sound signals.
It has been proposed to transmit a television signal having stereo audio components. An L-R difference signal to be transmitted, e.g., as an amplitude modulated (AM) suppressed carrier at, e.g., twice the horizontal scanning frequency. To maintain compatibility with existing monophonic (mono) receivers, the monaural signal, which consists of the L+R sum signal, is to be transmitted as a frequency modulated (FM) carrier in the conventional fashion. In a television receiver, the L+R and L-R signals are demodulated in a television receiver and then matrixed (added) to produce L and R signals. It has also been proposed to transmit an audio signal for a so called "second audio program" (SAP), e.g., for providing the main program in a second language, using, e.g., a frequency modulated carrier at four times the horizontal frequency.
For both the L-R and SAP signals it is also proposed to use a so called "dbx" companding system in which variable preemphasis (compression) of high frequency components at the transmitter and complementary deemphasis (expansion) at the receiver is used in order to improve the signal-to-noise (S/N) ratio of the corresponding reproduced signals. A companding system is not used for the L+R signal since, if it were, the ability of existing receivers to reproduce monaural signals would be disturbed. At any rate, the L+R signal is less subject to being contaminated by interference than the L-R or SAP signals because its carrier is at a much lower frequency. The "dbx" companding system is described in the publication entitled "Compandor Complexity Analyses" published by the Broadcast Television Systems Committee of the Electronic Industries Association. The purpose of providing substantial high frequency content in the transmitted signal is that it has been found that the greater the high frequency content of a transmitted signal the less perceptible will be noise (i.e., the more noise will be masked). Unfortunately, it has been found by the present inventor that due to the presence of noise under certain conditions, as will be explained below in detail, the deemphasis in the receiver will not match or be complementary to the preemphasis in the transmitter. Thus, the received audio signal will not be restored to its original level. In addition, the phase shifts caused by the preemphasis and deemphasis circuits will also no longer be complementary. In the case of stereo reproduction, this results in reduced stereo separation since the L-R and L+R signals when matrixed will not combine to produce the proper L and R signals.